MEDIAN NERVE ANATOMY
The Laborer's Nerve (Coarse Power and Sensation)
COMPRESSION SITES
Critical Must-Knows
- Formed by Lateral (C5-7) and Medial (C8-T1) Cords
- Runs MEDIAL to Brachial Artery in Cubital Fossa (TAN: Tendon, Artery, Nerve)
- Supplies Flexor Compartment of Forearm (Except FCU + Ulnar 1/2 FDP)
- Supplies LOAF muscles in hand (Lumb 1/2, Opponens, APB, FPB-sup)
- Sensation to Radial 3.5 digits (Palmar)
Examiner's Pearls
- "Hand of Benediction (High Median) is an ACTIVE sign (trying to fist)
- "Ulnar Claw (Low Ulnar) is a PASSIVE sign (at rest)
- "Palmar Cutaneous Branch is SPARED in Carpal Tunnel (arises proximal)
- "AIN is purely MOTOR (No sensory loss, just 'OK' sign fail)
Clinical Imaging
Imaging Gallery
The 'Hand of Benediction' Trap
Hand of Benediction
High Median Palsy. Patient tries to make a fist.
- Index/Middle FDP + FDS paralyzed → Cannot flex.
- Ring/Little FDP intact (Ulnar) → Flex.
- Result: Index/Middle straight, others flexed. Active Sign.
Ulnar Claw
Low Ulnar Palsy. Patient at rest.
- Intrinsics paralyzed (Lumb 3/4, Interossei).
- MCPs hyperextend (EDC), IPs flex (FDP).
- Result: Ring/Little clawed. Passive Sign.
| Nerve/Branch | Motor Function | Sensory Area | Key Sign |
|---|---|---|---|
| Main Median (High) | Pronator, FCR, FDS, PL | Palm + Digits | Benediction Hand |
| AIN (Forearm) | FPL, FDP (Idx/Mid), PQ | None (Joint prop only) | Cannot make 'OK' sign |
| Palmar Cutaneous | None | Thenar Eminence / Palm | Spared in CTS |
| Recurrent (Hand) | LOAF Muscles | None | Thenar Wasting |
PACSubtypes of Median Nerve Injury
Memory Hook:PAC-Man eats the median nerve.
TANCubital Fossa Contents
Memory Hook:TAN your arm from Lateral to Medial.
LOAFThenar Muscles (LOAF)
Memory Hook:The loaf of bread in your hand.
Overview
The Median Nerve is the "Eye of the Hand" (sensory to thumb/index) and the "Laborer's Nerve" (power grip via FDS/FDP/Thenar).
Neurovascular
Axilla & Arm
- Formed effectively by the fusion of Lateral (C5-7) and Medial (C8-T1) cords.
- No branches in the arm.
- Runs with Brachial Artery. First lateral, then crosses to medial.
Elbow (Cubital Fossa)
- Passes under the Bicipital Aponeurosis (Lacertus Fibrosus).
- Compression site: Lacertus Syndrome.
- Lies Medial to the Brachial Artery ("TAN").
- Enters forearm between the two heads of Pronator Teres.
- Compression site: Pronator Syndrome.
The nerve is vulnerable at these multiple fibrous arches.
The nerve is vulnerable at these multiple fibrous arches.
The Lacertus Fibrosus (Bicipital Aponeurosis) originates from the Biceps tendon and inserts into the ulna. It covers the median nerve and brachial artery. In bodybuilders or laborers, a thickened lacertus can compress the nerve, mimicking pronator syndrome.
Branching Order (Proximal to Distal)
| Branch/Structure | Level | Function | Clinical Relevance |
|---|---|---|---|
| No Branches | Arm | None | High palsy spares nothing below |
| Pronator Teres | Elbow | Pronation | First motor branch |
| FCR | Proxi Forearm | Wrist Flexion | Tendon transfer donor |
| Palmaris Longus | Proxi Forearm | Fascia tensor | Graft harvest |
| FDS | Mid Forearm | PIP Flexion | Independent function |
| AIN (FPL/FDP/PQ) | Mid Forearm | DIP/IP Flexion | OK Sign / Pinch |
| Palmar Cutaneous | Distal Forearm | Palm Sensation | Spared in CTS |
| Recurrent Motor | Hand (Tunnel) | Thenar Motor | Million Dollar Nerve |
| Digital Sensory | Hand | Sensation 3.5 | Numbness in CTS |
Classification Systems
Carpal Tunnel Syndrome Severity (Neurophysiology)
| Grade | Sensory (SNAP) | Motor (CMAP) | EMG |
|---|---|---|---|
| Mild | Slowed / Reduced Amp | Normal | Normal |
| Moderate | Absent / Severe Slowing | Delayed Latency | Normal / Mild changes |
| Severe | Absent | Reduced Amp / Absent | Denervation (Fibs/Pos waves) |
Clinical Assessment
Phalen's Test
- Wrist flexion for 60 seconds.
- Compresses nerve.
- Positive if paresthesia reproduced in digits.
- Reverse Phalen's: Wrist extension (increases pressure even more).
Tinel's Sign
- Percussion over nerve.
- Wrist: CTS.
- Proximal Forearm/Elbow: Pronator Syndrome.
Durkan's Compression
- Direct compression over carpal tunnel for 30s.
- Most sensitive test for CTS.
OK Sign (Kiloh-Nevin)
- Ask patient to make an 'O' with thumb and index.
- Normal: Tip-to-Tip pinch (FPL + FDP active).
- AIN Palsy: Pulp-to-Pulp pinch (Posterior pinch). FPL/FDP failed, Adductor/FDS compensate.
Differential Diagnosis Matrix
| Condition | Night Pain | Sensory Loss | Motor Weakness |
|---|---|---|---|
| Carpal Tunnel | Yes (Classic) | Digits 1-3.5 | APB (Thenar) |
| Pronator Syn | Rare (Activity related) | Palm + Digits | FPL/FDP/APB |
| AIN Syndrome | Deep forearm ache | None | FPL/FDP (OK sign) |
| C6 Radiculopathy | Neck pain | Thumb/Index (Dermatome) | Biceps/Wrist Ext |
Examination Pearls
Motor Testing (Detailed)
- OK Sign (AIN): FPL/FDP. Look for "tear drop" (Pulp-to-Pulp).
- Abduction (APB): "Touch the ceiling". Palpate muscle belly to exclude trick movement (PL/EPL).
- Opposition (Opponens): "Touch pin to thumb".
- FDS Test: Hold other fingers in extension.
- FDP Test: Hold PIP in extension.
Sensory Maps
- Autonomous Zone: Tip of Index Finger.
- Splitting: Radial 1/2 of Ring Finger.
- Palm: Proximal to wrist crease (PCB).
Provocative Tests Sensitivity
| Test | Sensitivity | Specificity | Notes |
|---|---|---|---|
| Durkan's | 89% | 90% | Most accurate manual test |
| Phalen's | 68% | 73% | Less specific in elderly |
| Tinel's | 50% | 77% | Great for tracking axon regeneration |
Investigations
Nerve Conduction Studies
- Indications: atypical symptoms, atrophy, revision, workers comp.
- Findings:
- Increased distal motor latency (greater than 4.2ms).
- Decreased sensory conduction velocity.
- Comparison: Compare to Ulnar/Radial (4th digit double innervation) to rule out polyneuropathy.
NCS is the gold standard for grading severity.
Management Strategy
Carpal Tunnel Syndrome
- Conservative:
- Night splints (neutral).
- Steroid injection (Diagnostic & Therapeutic).
- Ergonomics.
- Surgical:
- Indications: Failure of conservative, constant numbness, thenar wasting.
- Procedure: Carpal Tunnel Release (Open or Endoscopic).
Surgery is highly effective for night symptoms.
Management Algorithm
| Scenario | First Line | Second Line | Surgery Indication |
|---|---|---|---|
| Mild CTS (Nocturnal only) | Splint / NSAIDs | Steroid Injection | Failed conservative over 3m |
| Moderate CTS (Sensory loss) | Injection + Splint | Consider early surgery | Patient preference / Failure |
| Severe CTS (Wasting) | Surgery (Release) | None | Relative Emergency (prevent permanent loss) |
| Acute CTS (Trauma) | Reduction of fracture | Release if persistent | Compartment Syndrome equivalent |
Surgical Technique
Open Carpal Tunnel Release
Procedure Steps
Incision in line with ring finger axis. Distal to Kaplan's Cardinal Line. Avoid Palmer Cutaneous Branch (ulnar to PL tendon).
Incise Palmar Fascia. Identify distal edge of Transverse Carpal Ligament (TCL).
Divide TCL ulnarly to protect Recurrent Branch (radial). Visualize fat pad distally (Superficial Palmar Arch). Release proximally into forearm fascia.
The Million Dollar Nerve
The Recurrent Motor Branch has variable anatomy. In 50% it is Extraligamentous. In ~30% Subligamentous. In ~20% Transligamentous (goes THROUGH the ligament). Always cut the ligament on the ULNAR side.
Meticulous hemostasis is required to prevent hematoma and scarring.
Complications
| Complication | Cause | Management |
|---|---|---|
| Pillar Pain | Loss of arch support/ligament healing | Time, padding (Resolves by 6m) |
| Incomplete Release | Flexor retinaculum intact proximally | Revision |
| Recurrent Branch Injury | Radial side incision | Repair / Tendon Transfer |
| CRPS | Nerve injury / idiopathis | Multimodal therapy |
Rehabilitation
- Splinting: Generally NOT required for simple CTR.
- Motion: Immediate finger and wrist ROM.
- Strengthening: Grip strengthening at 4-6 weeks.
- Return to work: Desk (1-2 weeks), Light Manual (4 weeks), Heavy (6-8 weeks).
Early mobilization reduces complex regional pain syndrome risk.
Outcomes
- Success: 90% good/excellent results for CTS release.
- Night Pain: Resolves almost immediately ("Best sleep in years").
- Numbness: Variable recovery. Permanent if long-standing.
- Recurrence: Rare (less than 5%). Look for scarring, missed diagnosis, or double crush.
Special Scenarios
Martin-Gruber Anastomosis
-
Median → Ulnar connection in forearm.
-
15% prevalence.
-
Can cause "Ulnar" muscles to be spared in a high Ulnar lesion (supplied by Median).
-
Ulnar → Median connection in hand (Deep branch to Recurrent).
-
Can cause Thenar sparing in CTS.
These anomalies can confuse NCS findings.
Evidence Base
Endoscopic vs Open CTR
- No long term difference in symptoms or severity
- Endoscopic has earlier return to work (approx 1 week)
- Endoscopic has transiently higher nerve injury rate (though rare)
Corticosteroid Injection Efficacy
- Significant short-term benefit (less than 1 month)
- No significant benefit beyond 1 year
- Predictor of surgical success
Ultrasound vs NCS for CTS
- Ultrasound sensitivity 85%, specificity 90%
- NCS sensitivity 90%, specificity 95%
- US is a valid alternative but NCS remains gold standard
Surgical Treatment of Pronator Syndrome
- 80% patient satisfaction after release
- Identify and release all 4 compression sites
- Long incision required
AIN Syndrome Recovery
- Spontaneous recovery in 90% of cases
- Time to recovery 6-12 months
- Wait at least 12 months before surgery
Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Thenar Wasting
"A 65-year-old lady presents with severe thenar wasting but no pain. She says her hands just feel 'clumsy'. Diagnosis?"
Scenario 2: Failed CTR
"A patient returns 3 months after Open CTR with WORSE pain and pillar tenderness. What is your differential?"
Scenario 3: AIN Palsy
"A novice gymnast presents with inability to flex the IPJ of the thumb and DIPJ of the index finger. There is no sensory loss. What is the pathology?"
MCQ Practice Points
Martin-Gruber
Q: Which fibers cross in Martin-Gruber anastomosis? A: Motor fibers from Median to Ulnar. Usually AIN branch fibers crossing to Ulnar nerve in the forearm to supply intrinsics (First Dorsal Interosseous).
Lumbaricals
Q: Which lumbricals are Median innervated? A: 1 and 2 (Index and Middle). They are unipennate. 3 and 4 are Ulnar and bipennate.
CTS Anatomy
Q: Which structure is most superficial in the Carpal Tunnel? A: FDS to Middle/Ring. The FDS tendons are stacked 2 over 2 (3/4 over 2/5). The FPL is radial/deep. The Nerve is superficial to the tendons.
Pronator Teres Heads
Q: The Median nerve passes between which two heads? A: The superficial (humeral) and deep (ulnar) heads of Pronator Teres. The ulnar artery passes deep to the deep head (separating artery and nerve).
Palmar Cutaneous Branch
Q: Where does the Palmar Cutaneous Branch arise? A: ~5cm proximal to the wrist crease. It travels superficial to the Transverse Carpal Ligament. This is why it is spared in Carpal Tunnel Syndrome (compression is deep to ligament) but can be injured in the surgical incision if placed too radially.
Australian Context
- WorkCover: CTS is a designated disease in some states for repetitive tasks.
- Guidelines: Endoscopic release is popular in private practice; Open is standard in public teaching hospitals.
- Rural: GP proceduralists often perform Open CTR in rural Australia.
High-Yield Exam Summary
Anatomy High Yield
- •Roots: C5-T1
- •Cords: Medial + Lateral
- •Tunel: 9 Tendons + 1 Nerve
- •LOAF: Lumbricals 1/2, Opponens, APB, FPB
Clinical Signs
- •Benediction: High Palsy (Active)
- •OK Sign: AIN (Motor)
- •Phalen's: CTS (Sensory)
- •Durkan's: CTS (Compression)
Key Numbers
- •6mm: Normal 2-point discrimination
- •Greater than 10mm²: US diagnosis of CTS
- •Greater than 4.2ms: Motor Latency (abnormal)
- •5cm: PCB branching proximal to wrist
Surgical Steps
- •Incision: Ring finger axis
- •Kaplan's Line: Distal extent
- •TCL: Cut on Ulnar side
- •Protect: Recurrent Branch
Rehabilitation Protocol
| Phase | Timeframe | Goals | Precautions |
|---|---|---|---|
| Acute | 0-2 Weeks | Wound healing, Edema mgt | Avoid heavy grip |
| ROM | 2-6 Weeks | Tendon gliding, Scar massage | Desensitization |
| Strengthening | 6+ Weeks | Work hardening, Grip strength | Return to sport |
Specific Exercises
- Nerve Gliding: "Median Nerve Flossing".
- Position 1: Fist.
- Position 2: Extend fingers.
- Position 3: Extend wrist.
- Position 4: Supinate.
- Position 5: Stretch thumb.
- Tendon Gliding: Hook fist, Straight fist, Full fist.